Magnetic Microbes

by Sandi Clement

In nature, organisms use a variety of methods to figure out where
they are and where they need to be going. Plants use sunlight and the
force of gravity to determine which direction they should grow, birds
migrate in part using the sun and stars to guide their path, and we
humans can simply ask for directions at the nearest gas station. Some
organisms use the earth's geomagnetic field to orient themselves and
navigate through their environment. Organisms that use the earth's
geomagnetic field have some type of internal compass. The smallest
organisms that use this navigational method are called
magnetotactic bacteria.

Magnetotactic bacteria were discovered in 1975 by Richard
P. Blakemore. Blakemore noticed that some of the bacteria
that he observed under a microscope always moved to the same
side of the slide. If he held a magnet near the slide, the
bacteria would move towards the north end of the magnet.
These bacteria are able to do this because they make tiny,
iron-containing, magnetic particles. Each of these particles
is a magnet with a north pole and a south pole. The bacteria
arrange these tiny magnets in a line to make one long
magnet. They use this magnet as a compass to align
themselves to the earth's geomagnetic field.

Why would these bacteria need a compass? Like many other types of
bacteria, magnetotactic bacteria don't like oxygen very much. They
will move away from areas with high oxygen and toward areas with low
or no oxygen. In an aquatic environment, the level of oxygen
decreases as one moves deeper into the water. So, magnetotactic
bacteria like to live in the deeper parts of their aquatic
environments. They use their magnetic compass to tell them which way
is down.

How do they do this? It has to do with the direction of the
geomagnetic field. In the Northern Hemisphere; the geomagnetic north
actually points down at an angle. So, magnetotactic bacteria that are
aligned to this field are also pointing down. By moving north along
this field, they move deeper into the water, and into areas with less
oxygen. Interestingly, in the Southern Hemisphere, the geomagnetic
north actually points up and at an angle. So, magnetotactic bacteria
in this half of the world are "south-seeking", which points them
downward. At the equator, the geomagnetic north doesn't point up or
down, so the magnetotactic bacteria found there are a mixture of
north-seeking and south-seeking bacteria.

Scientists are also interested in practical applications involving
these magnetic microbes. While it isn't likely we'll be using these
bacteria to stick notes to our refrigerators, they could prove to be
useful to humans. The tiny magnets that these simple organisms make
are far superior to those produced by people. So, scientists and
engineers are trying to develop ways to use this magnetic material in
places where tiny magnets are much better than big magnets.